Sliding brake discs are normally received on a sleeve, hub or the like by means of teeth, splines or the like, or by any other means giving a fixed connection in rotational direction but a sliding connection in axial direction. The hub, sleeve etc. is received on and connected to the wheel axle. Depending on type and make of the brake the brake disc may either be received on a sleeve on the hub, connected to the wheel axle, or the brake disc may be received directly on the hub, without the use of any sleeve. In order to give sliding there has to be a certain play between the teeth, splines or the like on the inner circumference of the brake disc and the teeth, splines or the like on the outer circumference of the hub or sleeve.
In a released condition the disc is free to move in axial direction to some extent.
If, in released condition, the brake disc is put in an inclined position in relation to the rotational axis of the hub or sleeve, the disc will be translated in an axial direction by influence of wheel rotation. The direction of travel is dictated by the direction of the inclination and the direction of wheel rotation. Inclination of a brake disc in released condition is inevitable and occurs randomly in most brakes of this type. The maximal inclination of each brake disc is generally limited either by the distance between the brake pads on both sides of the brake disc or the play in the teeth or splines connection. The brake disc may normally travel until it comes into contact with a brake pad. Such a contact between brake pad and brake disc in a released condition causes an unwanted braking action when the wheel rotates normally called dragging. Dragging may not lead to any serious problems regarding the driving of the vehicle, however, heating of the disc and brake pad, wear of pad and disc, extra energy consumption etc. may occur. Thus, the longevity of the brake pads, and possibly the brake as such, may be negatively influenced by dragging.
In the contact with the brake pad in the released condition there is a risk that the brake disc may be locked in a position in direct contact with the brake pad. This effect is often referred to as self-locking. The driver will not observe when the brake is applied by self-locking, and it may lead to serious damages of the brake and possibly the wheel assembly. Furthermore, if a brake is applied suddenly and unexpected by self-locking it may lead to a hazardous situation. The risk of self-locking may be influenced by a number of factors, such as the radial play between hub and disc, the distance between the brake pads, the position of the brake pads, the rotational direction of the hub etc.
If and as soon as the brake disc is in a vertical position, i.e. in right angles to the sleeve or hub, it may not travel axially along the hub or sleeve.